Despite a $900,000 mistake found during a routine inspection of bridge beams, North Texas Tollway Authority officials said they plan to finish the Chisholm Trail Parkway by late September.

“The beam had a flaw in it. These were not designed properly,” said Elizabeth Mow, the authority’s assistant executive director for infrastructure.

Last week, the authority’s board agreed to pay the $900,000 for the new bridge beams, which are being installed. The agency will ask for its money back from the contractors who should have caught the error before the faulty beams arrived, she said.

The 28-mile toll road, stretching from near downtown Fort Worth to U.S. 67 in Cleburne, opened in May, but key connections at the ramps to Interstate 30 near downtown and Interstate 20 in southwest Fort Worth remain under construction.

The beams in question were installed on bridges spanning the Trinity River and University Drive west of downtown Fort Worth, areas that had not been opened, officials said. The beams were not strong enough.

The problem involved the ability of the beams to withstand shear, an engineering term that describes when key parts of a structure deform or bend. The beams are I-shaped, with the horizontal portion, called a flange, and the vertical portion, or web, working together. The web was the primary issue in the design flaw, Mow said.

For now, motorists can use most of the roadway, with a few exceptions.

Northbound traffic connects directly with eastbound I-30 traffic as well as Summit Avenue, Cherry Street and Lancaster Avenue — making it relatively easy to get to downtown Fort Worth. But there are no direct connections yet for downtown motorists who want to get on the southbound parkway. Drivers typically take I-30 to Montgomery Street to reach the nearest on-ramp.

A westbound I-20 connection to the southbound parkway opened in July, and a connection from the northbound parkway to eastbound I-20 opened in June. But two other I-20 connections may be under construction through late September.

Some motorists said it appears to them that the parkway is not being used by large numbers of commuters. But tollway authority officials said plenty of drivers are using it. In fact, revenue is ahead of projections, Horatio Porter, the agency’s chief financial officer, told board members last week.

The Chisholm Trail Parkway and the western extension of the President George Bush Turnpike were financed together in a complicated deal reached several years ago, so Porter’s figures included revenue generated from tolls collected on both roads.

Through the first six months of the year, revenue on the two roads was $31.4 million, significantly higher than $28.1 million budgeted, he said. The same period a year earlier —before the parkway opened — the revenue was $20.4 million, he said.

“We continue to see real positive results with Chisholm Trail,” Porter said. “With just two months under our belt, that particular facility has generated $625,000 more than we anticipated, so it’s ramping up quite impressively.”

Revenue for the authority’s whole system, including toll roads in Collin, Dallas and Denton counties, was $293 million for the first six months of the year, he said, 12 percent ahead of the same period a year ago.